1. Field of the Invention
This invention relates generally to treatments for wells and/or subterranean formations and, more specifically, to well treatments employing polymer-containing fluids. In particular, this invention relates to introduction of an enzyme and/or oxidative-breaker-containing pre-treatment fluid or pre-flush into a wellbore and/or subterranean formation prior to introduction of a polymer-containing treatment fluid into the well.
2. Description of Related Art
Filter cakes or face plugs may be formed during various procedures performed within a wellbore. Typically, filter cakes are composed of precipitates, such as silicates formed from drilling muds, or residue formed after using polymer-containing gelatable fluids. In such cases the residue may contain polysaccharides.
For example, during hydraulic fracturing, a fluid is typically injected into a wellbore and subterranean formation under relatively high pressure. Once the natural reservoir pressures are exceeded, the fracturing fluid initiates a fracture in the formation which generally continues to grow during pumping. Viscosity of the treatment fluid generally affects the fracture length and width. Increased viscosity is typically obtained by the gelation of suitable polymers, such as polysaccharides. A gel fluid may be accompanied by a propping agent, which results in the placement of the propping agent within the fracture thus produced. The proppant remains in the produced fracture to prevent complete closure of the fracture and to form a conductive channel extending from the wellbore into the formation being treated once the fracture fluid is recovered.
In other cases, polymer-containing gelatable treatment fluids may be employed during drilling, completion, remedial and/or workover operations. During such operations, polymer-containing treatment fluids may be employed to facilitate, for example, drilling, cleanout, solids transport, fluid loss control, etc. For example, polysaccharides may be used to thicken fluids and control fluid loss. Polysaccharides may also be used to support proppants, such as in sand control fluids and completion fluids. Specific examples of polymer-containing treatment fluids include, but are not limited to, drill-in fluids, drilling muds, gravel pack fluids, gelled acid fluids, foamed cleanout fluids, lost circulation pills, etc.
Typically, at the end of fracturing or other wellbore operations which employ polymer-containing treatment fluids, a polymer gel is degraded and the treatment fluids recovered. Recovery of fracturing and other polymer-containing treatment fluids is typically accomplished by reducing the viscosity of the fluid to a low value such that it may flow naturally from the formation under the influence of formation fluids and pressure. This viscosity reduction or conversion is typically referred to as "breaking". Enzymes are commonly employed as breakers due to inherent specificity and the infinite polymer degrading activity of enzymes. The use of enzyme breakers allows the degradation of the polymeric gelling agents in the fluid to proceed in controllable manner to reduce the fracturing fluid viscosity by cleavage of the polymer backbone into fragments which tend to remain soluble in the aqueous-based fluid. Other types of breakers include oxidative breakers.
During procedures employing polymer-containing well treatment fluids, filter cakes are often formed. Filter cakes may form in a variety of ways. For example, when gelled fluids are pumped into a subterranean formation or a wellbore penetrating a subterranean formation, fluid may leak into the formation matrix through the pore spaces of the rock. The pores tend to act as filters, permitting fluid to leak into the rock matrix while filtering out the gel. When this occurs, a layer of filtered gel may deposit on the face of the matrix and plug the formation. Incomplete gel degradation is another example.
When present, filter cakes or gel damage tend to fill pores in the rock matrix and curtail the flow of fluids from or into the formation matrix. A polymeric filter cake may be characterized as a dense mass of polymer deposited on a formation face by dynamic fluid loss to the formation when a polymeric-containing treatment fluid is exposed to the formation face. In the case of hydraulic fracture treatments, such filter cakes may be concentrated within a proppant pack and/or nearby flow channels by fracture width reduction upon fracture closure. In this regard, polymers used as gelling agents in fracture treatments are typically too large to penetrate the rock matrix, therefore, they tend to be concentrated within the fracture. The filter cake may then plug the flow channels, thereby reducing the flow of fluids during production and/or injection. Studies have documented that the polymer concentration within a fracture may be as much as 20-fold the surface gelling agent concentration. Although some polymers may not form filter cakes, the viscosity of these fluids creates damaging conditions analogous to those found with filter cakes. Therefore, the term "filter cake" when used as a generic term in this disclosure may also refer to these conditions.
Polymeric damage in the form of unbroken gel residue or dynamically formed filter cake may significantly reduce well productivity and/or injectivity. Gel residue damage may be characterized as the blockage of pore throats by an unbroken viscous gel having limited mobility, or by insoluble polymer fragments. For example, in the case of induced fractures, the degree of damage is typically proportional to the amount of fracture pore volume occupied by gel residue.
During polymer-containing well treatments, such as hydraulic fracturing treatments, breakers are typically added to the polymer treatment fluid to reduce the viscosity of the polymer treatment fluid so that the polymeric materials may be recovered. Such recovery tends to reduce polymeric damage by, for example, minimization of the amount of gel residue remaining in a fracture after load recovery. However, concentration of breaker which may be employed in a polymeric-containing fluid such as a fracturing fluid must typically be limited due to the effect of the breaker on the viscosity of the fluid during treatment. Consequently, complete or near complete degradation of a filter cake and/or unbroken gel residue may not be possible. Furthermore, during recovery of a polymer-containing treatment fluid from a subterranean formation, reservoir fluids (such as oil, gas or water) may displace portions of the treatment fluid containing breaker while leaving behind filter cake and/or undegraded gel residue. Such filter cake and/or unbroken gel residue then tends to interfere with conductivity of the subterranean formation.
In the case of well treatment fluids which do not employ breakers, the removal of filter cake and/or unbroken gel residue is typically not possible without the performance of a follow-up treatment employing a polymer degrading material such as an enzyme and/or oxidated breaker. However, the effectiveness of such treatments may be limited by accessibility of the filter cake and/or unbroken gel residue damage to an enzyme or oxidated breaker treatment. Because such polymeric damage tends to plug the formation, such remedial filter cake removal treatments may tend to contact and/or enter the subterranean formation at only selected locations, bypassing other areas of damage.